We have shown previously that fast-twitch skeletal muscle surgically isolated by removal of synergistic muscles undergoes varying degrees of enlargement. Accompanying this hypertrophy are reductions in the enzymatic activity and functional properties of (1) Ca ions regulation of myofibril ATPase and myosin ATPase; (2) Ca ions uptake capacity of the sarcoplasmic reticulum, a vesicle system known to sequester Ca ions during contraction; ana (3) activity levels of enzymes involved in the pathway of anaerobic synthesis of ATP. These subcellular changes should alter the functional properties of the enlarged muscles in terms of contractile and relaxation speed and endogenous utilization of phosphocreatine and glycogen. Studies will be undertaken to test the mechanical and anaerobic properties of enlarged skelatal muscle contractions in situ. These tests will ascertain the extent that compensatory stressed fast-twitch muscle is converted biochemically and functionally into a slow-twitch muscle.